Fix Jacoco to work with build cache Jacoco code coverage should work well with the build cache out of the box. Since appending to a coverage file works with parallel test execution, see https://github.com/jacoco/jacoco/pull/52, we set `append=true` and delete the coverage data just before the test task starts to execute.

Note that this is a breaking change: separate tasks now cannot use the same coverage file, since each of the tasks will delete it.